Arsenate [As(Ⅴ)] reducing microorganism plays an important role in regulating As redox transformation and mobility.The motivation of this study was to compare different As reducing mechanisms on the As biotransformation.In this work,Pantoea sp.IMH,Alkaliphilus oremlandii strain OhILAs,and Shewanella oneidensis MR-1 were employed to investigate their contributions to the As biotransformation in the waste residue,which contains high concentration of As,iron(Fe),and SO42-.The results showed that Pantoea sp.IMH with arsC gene could aerobically reduce 100%As(Ⅴ)released from the waste residue.However,strain IMH did not enhance the total As release compared with the control.Strain OhILAs possessing arrA gene,could anaerobically reduce dissolved As(Ⅴ)and could not enhance As release.For strain MR-1,an iron reducer which also harbors arsC gene,could significantly enhance the As mobilization from the solid residue.However,As(Ⅴ)was the predominant species because strain MR-1 could not respire As(Ⅴ).The correlation relationship analysis between iron and As concentrations(p<0.01)indicated that iron reduction contributed to As mobility.The As K-edge XANES results showed that As(Ⅲ)ratio increased when three bacteria were added.Moreover,strain MR-1 could reduce dissolved sulfate in the waste residue to sulfide and H2S.Sulfide concentration decreased with increasing time due to the formation of FeS2 in the solid phase,and consequently affect As speciation.In conclusion,these three different bacteria play different roles in the As biotransformation.The arsC gene owners mainly control the As speciation in the aqueous phase in the aerobic environment,while in the anaerobic condition,As speciation should be regulated by arrA gene owners and its mobility is greatly enhanced by the iron reduction.